While the intrathymic development of T lymphocytes via positive and negative selection has been well characterized over the past 2 decades, the processes which regulate the development of pro-thymocytes and immature thymocytes from hematopoietic stem cells (HSC) is poorly understood. Recently an intermediate stage of HSC->lymphoid differentiation, interleukin-7 (IL-7) and c-kit ligand (KL). IL-7 is a stromal derived cytokine which has anti-apoptotic proliferative and differentiative effects on lymphoid progenitors. The IL-7 receptor (IL-7R) consists of two of two non-covalently linked subunits, IL-7Ralpha and the common gamma chain (gamma/c), both of which are members of the Type I cytokine receptor superfamily. The IL-7Ralpha and gamma/c heterodimerize in the presence of IL-7, resulting in activation of multiple pathways, including the PI-3K, ras-MAPK, and Nak-Stat signal transduction pathways. The lymphopoietic defect in mice rendered IL-7-/- by homologous recombination is less severe than that of mice with IL- 4Ralpha-/- genotype. Kit ligand (KL, Stem cell Factor [SCF], Steel factor [SI]) is the ligand for c-kit, a receptor tyrosine expressed by HSC and immature progenitors, including CLP. C-kit also activates multiple pathways, including the Jak-Stat pathway. Although the defective HSC and erythroid compartments of mice with mutations of the c-kit or KL loci are well appreciated, the c-kit pathway also significantly contributes to lymphopoiesis. Mice with mutations of either KL or c-kit have defective thymopoiesis, which can be corrected by transplantation of a normal thymus or normal HSC, respectively. We have generated IL-7-/- Kit/w41/w41 double homozygous mice, which have more severely defective lymphopoiesis than either parental strain, suggesting an interaction between the IL-7R and c-kit signaling pathways. The hypothesis to be tested in the proposed studies is that the IL-7R and c-kit signaling pathways directly interact in lymphoid differentiation of primitive hematopoietic progenitors in a manner similar to the interactions of the erythropoietin receptor (EpoR) and c-kit receptor. The studies will investigate how interactions between the IL-7R and c-kit signaling occur. A major emphasis will be the identification of unique, redundant, or synergistic interactions of Stat molecules activated by the IL-7R (Stat5) and c-kit (Stat1). Changes in gene expression induced by each ligand alone or together will be studied. The mechanisms by which each pathway alter the differentiation of CLP will be characterized. Besides their relevance to lymphoid differentiation, the studies will provide a model for how cytokine receptor pathways, one involving a Type I cytokine receptor (IL-7R) and the other a receptor tyrosine kinase (c-kit), interact during HSC differentiation. Such interactions are also likely to be relevant to studies of erythroid and myeloid differentiation, where similar receptor interactions may occur.